Human rhinoviruses (HRVs) and encephalomyocarditis virus (EMCV) belong to different genera of the picornavirus family, but the translation of the RNAs of both viruses is by the same mechanism, that is, internal ribosome entry. In rabbit reticulocyte lysates this translation initiation is efficient for mRNAs bearing the EMCV 5′ untranslated region (5′ UTR), but very inefficient for mRNAs bearing the HRV 5′ UTR, unless factors from HeLa cells are added. The copurification of the HeLa cell translation stimulatory activity with proteins which can be specifically cross-linked to the HRV 5′ UTR by u.v. irradiation has been examined. Both the EMCV and HRV 5′ UTRs can be cross-linked to a 58/60K protein doublet present in HeLa cell extracts in higher amounts than in reticulocyte lysates, which is shown to be very similar, if not identical to the polypyrimidine tract binding protein (PTB) previously identified as a component of a multi-subunit complex necessary for pre-mRNA splicing. However, the activity in HeLa cell extracts that specifically stimulates translation initiation on mRNAs with the HRV 5′ UTR does not copurify with the majority of the 58/60K protein present in these extracts, but copurifies with a minor fraction of these proteins and with a 97K protein which can be cross-linked to the HRV 5′ UTR but not to the EMCV 5′ UTR, and which is absent from reticulocyte lysates. It is proposed that the specific translation initiation stimulatory activity found in HeLa cells is due to a high complex containing the 97K polypeptide and PTB.


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